In one known process, sulfur oxides, such as sulfur dioxide and sulfur trioxide, are removed from oxygen-containing industrial off-gases by use of a copper-containing acceptor for the sulfur compounds. According to the process, a reduced acceptor, which contains copper as metallic element, is contacted first with an oxygen-containing off-gas to oxidize the metallic copper to copper oxide. Only in the oxidic form is the copper acceptor capable of binding the sulfur oxides as sulfate by chemisorption. At the same time, sulfur dioxide is oxidized to sulfur trioxide.
After a given time, the acceptor, depending on the quantity by weight of copper supported on the acceptor carrier, will be fully or almost fully loaded and will need to be regenerated. For regeneration, reducing gases are used. Preferably, hydrogen- and/or carbon monoxide-containing gas mixtures containing 5-30 percent by volume of hydrogen in addition to inert components such as steam, nitrogen and/or carbon dioxide as diluent are employed. As a result of the regeneration, the sulfate on the acceptor is decomposed and the bound sulfur oxide is released as dioxide. A reduced acceptor remains, which contains metallic copper, and the acceptor can be re-used for the acceptance of sulfur oxides as described previously. The chemistry of the oxidation and reduction reactions involved in this process have been comprehensively discussed in Petroleum and Petrochemical International, July 1972, pages 44 and 45.
Nitrogen oxides, which include NO and NO.sub.2, and are customarily referred to as NOX, occur in all industrial off-gases obtained by combustion with air. They also occur in the off-gases from chemical plants, such as nitric acid plants. Nitrogen oxides are also known to be atmospheric pollutants, and their removal from off-gases is therefore desirable.
A process has already been proposed in which both sulfur dioxide and NOX are removed simultaneously from industrial off-gases by adding ammonia to such off-gases and passing the off-gases over a copper-containing acceptor in order to accept the sulfur oxides. This proposal is based on the observation that nitrogen oxides are reduced to free nitrogen with ammonia in the presence of a copper oxide-containing catalyst. The reactions involved in that process are generally represented by the following total equations: EQU 6 NO + 4 NH.sub.3 .fwdarw. 5 N.sub.2 + 6 H.sub.2 O (1) EQU 6 no.sub.2 + 8 nh.sub.3 .fwdarw. 7 n.sub.2 + 12 h.sub.2 o (2)
although nitrogen oxides can be converted into nitrogen to a degree of almost 90 percent or more by means of ammonia and a copper oxide-containing catalyst, the conversion of nitrogen oxides with the simultaneous removal of sulfur oxides by means of a copper-containing acceptor proves not to exceed 70 percent.
I have now found that the cause for this limited reduction is related to the peculiarities of the process for the removal of sulfur oxides by means of a copper-containing acceptor. More particularly, both the acceptance and the regeneration of the acceptor are carried out at temperatures which are not higher than 475.degree. C. Preferably, the temperature for both process steps is roughly 400.degree. C. It appears, however, that the said oxidation of the reduced acceptor in which the metallic copper is converted into copper oxide is accompanied by large heat generation. This oxidation reaction, which takes place at the moment the oxygen-containing off-gas is contacted with the reduced acceptor bed with a sharply defined reaction front. Simultaneously, a temperature peak of more than 500.degree. C (approx. 550.degree. C to 600.degree. C, depending on the copper content of the acceptor), passes through the acceptor bed.
Although the average bed temperature scarcely rises because of this temperature peak - in practice it has been found that when the flue gases are contacted with the reduced acceptor at a temperature of 400.degree. C, a temperature rise of 10.degree. to 30.degree. C can occur in the bed -- the temperature peak passing through the bed therefore causes local temperatures of 600.degree. C. If ammonia is added to the flue gas for the reduction of NOX, this high initial temperature at the first introduction following regeneration of the acceptor will contribute to cause the oxidation of ammonia and NOX will be additionally formed: EQU 4 NH.sub.3 + 5 O.sub.2 .sup.Cu-cat. 4 NO + 6 H.sub.2 O (3)
this inverted reaction, which takes place above 500.degree. C and in which the copper-containing acceptor acts as catalyst, is the cause of the NOX conversion not exceeding 70 percent.
Improvement may be accomplished by ensuring that during the initial phase of the desulfurization process, when the flue gases are contacted with the reduced catalyst, no ammonia is present in or added to the gases to be treated. In this context, it must be borne in mind that the first quantities of flue gas which have passed through the acceptor bed will be free of sulfur oxide but will still contain nitrogen oxides. Considering the quantity of nitrogen oxides slipped through, it will not be possible to achieve a conversion in excess of 90 percent in this manner either.